A pixel sensor comprises an array of pixel sensor cells that detects two dimensional signals. Pixel sensors include image sensors, which may convert a visual image to digital data that may be represented by a picture, i.e., an image frame. The pixel sensor cells are unit devices for the conversion of the two dimensional signals, which may be a visual image, into the digital data. A common type of pixel sensors includes image sensors employed in digital cameras and optical imaging devices. Such image sensors include charge-coupled devices (CCDs) or complementary metal oxide semiconductor (CMOS) image sensors.
While complementary metal oxide semiconductor (CMOS) image sensors have been more recently developed compared to the CCDs, CMOS image sensors provide an advantage of lower power consumption, smaller size, and faster data processing than CCDs as well as direct digital output that is not available in CCDs. Also, CMOS image sensors have lower manufacturing cost compared with the CCDs since many standard semiconductor manufacturing processes may be employed to manufacture CMOS image sensors. For these reasons, commercial employment of CMOS image sensors has been steadily increasing in recent years.
Prior art CMOS image sensors comprise a photosensitive diode including a p-n junction between two differently doped semiconductor regions. When a photon impinges on the photosensitive diode, the photosensitive diode may generate an electron-hole pair if the photon interacts with the band structure of the semiconductor material comprising the photosensitive diode. The energy of the photon that induces electron-hole pair generation depends on the band gap of the semiconductor material. The wider the band gap, the greater the energy of a photon that is required to generate an electron-hole pair. For example, the wavelength range of photons for photogeneration of an electron-hole pair is from about 190 nm to about 1,100 nm for silicon, from about 400 nm to about 1,700 nm for germanium, and from about 800 nm to about 2,600 nm for indium gallium arsenide, respectively. Practically, due to the loss of efficiency of photogeneration near the edge of the wavelength ranges, usable wavelength ranges for detection of light may be substantially narrower than the wavelength ranges described above for each semiconductor material.
The choice of the semiconductor material for the photosensitive diode determines the wavelength window for light detection for the prior art CMOS image sensors. For example, a prior art CMOS image sensor employing silicon in a photosensitive diode is capable of detecting light in the visible spectrum range. Likewise, a prior art CMOS image sensor employing germanium in a photodiode is capable of detecting infrared light. Thus, the detection range of the prior art CMOS image sensor is limited to the wavelength range of the material employed in the photosensitive diode.
Some applications require, however, extended detection range that is beyond the range of wavelengths encompassed by a single semiconductor material. For example, optical sensors that require detection of visible wavelength spectrum range and infrared wavelength range cannot be provided either by a silicon based photosensitive diode or by a germanium based photosensitive diode.
In view of the above, there exists a need for a semiconductor structure including an optical sensor capable of detecting light over a wavelength range that extends beyond the detection range of a single semiconductor material, and design structures for the same.